The Chicago park shooting that left 13 people wounded last week was retaliation for a graze wound one of the alleged gunmen suffered in a separate shooting, the city's police superintendent said.

Bryon Champ, 21, charged this week with allegedly firing a gun into a crowd last Thursday night at Cornell Square Park, suffered the wound to his leg, Supt. Garry McCarthy said. Prosecutors said Champ was shot by rival gang members.

"The motivation for this incident was an unreported shooting where this individual, Mr. Champ, suffered a graze wound earlier last Thursday," McCarthy said Tuesday. "And as a result the retaliation took place."

Prosecutors said Champ told the other three about his injury and they went to the park with the intent to seek revenge for Champ being shot.

Witnesses said gunmen fired at least 20 shots at a group shortly before 10:15 p.m., and the spray of bullets sent a mass of people to the ground on the basketball courts. Among the injured was a 3-year-old boy, who was struck in the jaw.

All men were held without bond, and McCarthy said more charges are likely against the four men accused in the incident.

McCarthy said Champ was previously convicted of unlawful use of a weapon by a felon, which is "a crime that's designed to put dangerous criminals in jail, and he received boot camp as his punishment."

"A little more than a year later, he's the main player in a shooting of 13 people," he said.

McCarthy again called for a mandatory minimum sentence for illegal gun possession.

"To truly address violence for the long-term we need state and federal laws that keep illegal guns out of our communities and provide real punishment for the criminals who use them,” he said.